Mold for semi-conductor ingots



March 4, 1958 G. c. FLORIO MOLD FOR SEMI-CONDUCTOR INGOTS Filed Dec. 28, 1954 INVENTOR GERALD C, FL OR/O ATTGRNEY United States Patent 2,825,549 MOLD FOR SEMI-CONDUCTGR INGOTS Gerald C. Florio, Montclair, N. 1., assignor to International Telephone and Telegraph Corporation, New York, N. Y., a corporation of Maryland Application December 28, 1954, Serial No. 473,020

Claims. (Cl. 266-2) This invention relates to the manufacture of semiconductor ingots such as germanium ingots, and more particularly to a mold which is used in forming the ingots.

Germanium ingots are conveniently formed by placing a given amount of germanium oxide in a graphite mold, and reducing the germanium oxide by heat, to obtain germanium. The mold is preferably made of graphite because it is inert, easily purified and has practically no tendency to combine chemically with the germanium. The germanium ingots so formed, however, contain a certain percentage of impurities which are generally removed by passing the ingots through a series of induction heating coils. This method of purification is known as zone melting. As the ingot is run through the coil, the molten zone passes along the ingot, picking up and holding impurities in solution, thus carrying them to the end of the ingot where they solidify. Because of this mode of removing the impurities, there is a tendency for the ingot to increase progressively in cross-section from one end towards the other. Therefore, if an ingot of uniform cross-section was so treated, as is the conventional practice, the resulting purified ingot would be tapered throughout its length.

The tapered configuration is undesirable because the ingot which is to be used in the manufacture of electric crystals is again treated by adding a desired percentage of impurities uniformly throughout the ingot. If the tapered ingot is used, then the molten zone volume must be changed constantly or the impurities must be added in gradually increasing amounts from one end of the ingot towards the other in order to provide uniform distribution of impurities throughout the ingot. Regulating the application of impurities in this manner is expensive and troublesome.

Accordingly, it is an object of this invention to provide a mold of such configuration, that after the ingot of germanium is purified, its dimension throughout its length is substantially uniform.

in accordance with an aspect of the invention, there is provided a mold for forming a semi-conductor (germanium) ingot comprising an elongated vessel having sides, ends and a bottom defining a volume open at the top surface thereof, characterized in that the distance between the top and bottom of a vessel, wherein the germanium material is placed, gradually decreases from the front end towards the rear end. The front end is that end which passes through the induction coils first.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a cross-sectional side view of the mold;

Fig. 2 is a view taken along lines 2-2 of Fig. 1; and

Fig. 3 is a top view of the mold.

Referring now to the figures, there is illustrated a mold for forming two semi-conductor ingots. The mold is preferably made of graphite but may be made of any suitable material. The mold may be longer so that more than two ingots may be formed during one heat treatment but the invention loses its eflicacy to a considerable extent if the mold is made to form only one ingot; the

2,825,549 Patented Mar. 4, 1958 reason for this will be explained more fully hereinafter. The mold, as illustrated, comprises an elongated vessel having sides 1, ends 2, and a bottom 3, and may be considered as having two sections 4 and 5. The bottom 3 of the first section 4 is of gradually varying thickness which increases from one end towards the other of that section. Similarly, the bottom 3 of section 5 gradually increases in thickness from one end towards the other end thereof.

It is desirable that as many ingots be formed during one heat treatment as is economically possible. Usually the number so formed is determined by the length of the furnace. Most furnaces are capable of holding one ingot, corresponding in length to one of the sections of the mold, and have enough space to hold approximately another half ingot. Utilizing the mold of this invention permits the forming of two ingots during one heat treatment by building a simple extension onto the existing furnaces. It is to be realized that even with such an extension, it would be impossible to mold two ingots simultaneously, using conventional techniques, because the taper would be continuous over the length of the bar and at the smaller end the ingot would be an unsuitable dimension.

In accordance with the invention, the germanium material (not shown) is placed within the confines of the double mold as defined by the sides, ends and bottom thereof. The double mold is then placed in a furnace (not shown) and the germanium oxide material is melted, the molten material filling the hollow space 6 (Fig. 2) at the bottom of the vessel. Thus, the two ingots at this stage are approximately of the same dimensions, each tapered throughout its length, and the degree of taper being such that after purification each ingot is of uniform size throughout its length.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. A mold for forming a plurality of semi-conductor ingots comprising a plurality of similar sections, each having sides, ends and a bottom defining a volume open at the top thereof, characterized in that the bottom of each said section is inclined to the horizontal and the distance between the top and bottom of said vessel gradually decreases from one end towards the other.

2. The mold according to claim 1 wherein the bottom of each said section is of a thickness gradually varying from one end of the section towards the other.

3. The mold according to claim 1 wherein said mold comprises two similar sections.

4. The mold according to claim 1 wherein said mold is made of graphite.

5. A mold for forming a semi-conductor ingot comprising an elongated vessel having sides, ends and a bottom defining a volume open at the top thereof, characterized in that the bottom is inclined to the horizontal and the distance between the top and bottom of said vessel gradually decreases from one end towards the other.

References Cited in the file of this patent UNITED STATES PATENTS 2,615,010 Marinace Oct. 21, 1952 2,667,722 Jenkins Feb. 2, 1954 FOREIGN PATENTS 714,875 Great Britain Sept. 1, 1954 28,716 Great Britain Dec. 31, 1903 510,303 Belgium .muuuawqwu. Apr. 15, 1952 

